1. Field of the Invention
The invention relates to infrared instruments and cameras and in particular to calibration of the same.
2. Description of the Prior Art
Recent advances in infrared (IR) detector technology have led to the commercially availability of new detector technologies that do not require cryogenic cooling. These new detector technologies include ferroelectric and microbolometer technologies. The currently commercially available microbolometer detector arrays are based on a technology that was originally developed by Honeywell Sensor & System Development Center and was licensed to Boeing (formerly Rockwell), Raytheon (formerly Hughes' Santa Barbara Research Center) and British Aerospace (formerly Loral).
While these new technologies do not require cryogenic cooling, they are traditionally operated with a thermoelectric cooler to maintain a constant focal plane temperature usually between 0° to 20° C. The purpose of maintaining a constant detector temperature is that this will simplify the interpretation of the signal generated by the detector array and the generation of a thermal image. Since the detector is at a constant temperature, any measured voltage changes must correspond to thermal differences in the scene that is being imaged onto the focal plane array (focal plane array). The resulting simplification in processing is a result of assuming that the measured temperature differences and the temperature of focal plane array are constant. The apparent temperature from an object in the scene as compared to the average temperature of the scene at the focal plane array are small as compared to the average temperature of the scene (ΔT<<T). Assuming the temperature of the focal plane array is constant, then it can be assumed that the measured changes in voltage, that are the result of resistivity changes, are considered to be approximately linear as related to temperature. Since the transform is based on a linear relationship, then the slight differences in the responsivity of the individual elements in the array are typically corrected using a two point calibration that determines an offset and gain correction for each pixel. The results of this two point calibration are independent of the ambient temperature.
While there is a potential of using an uncooled microbolometer focal plane array without a thermoelectric cooler, the difficulty is that calculating the conversion between the measured voltage to a perceived scene temperature difference is now a more complex calculation. The relationship between the voltage and the temperature varies with the temperature of the focal plane array, and the specifics of this variation are different for every pixel in the array. Since the focal plane array temperature is unregulated, just viewing an IR scene will result in heating of the focal plane array.